System to Lower Emissions and Improve Energy Efficiency on Fossil Fuels and Bio-Fuels Combustion Systems

ABSTRACT

The present invention relates to a water heater system comprising: a hot water boiler having top, bottom and side walls; a flue connected to the top wall; a burner secured to a side wall of the first housing; a combustible feeder connected to the burner; an evaporator having a housing, the housing comprising an outlet, heat exchange elements located in the housing and a water discharge device spaced above the heat exchange elements, wherein the evaporator provides a source of humid air to the burner for increasing the combustion product dew point and to reduce NOx emission when burned; and a heat recovery system connected to the flue wherein the heat is used for heating water used by the water discharge device.

FIELD OF THE INVENTION

The present invention concerns a water heater system wherein the amount of water and the temperature of the air are closed loop controlled for reducing NOx production.

BACKGROUND OF THE INVENTION

In view of the rise in energy price, coal has become a cheap energy source in comparison to oil or natural gas. In hot water boilers, coal has been largely replaced by natural gas, electricity and oil as alternative fuel source, in part because coal is a heavy pollutant. However, because they are economical, the demand for hot water systems using coal burners has increased. Coal is economical, but generates amongst other things NOx, which are responsible for environmental problems like smog, acid rain, etc.

It is well known that by humidifying the intake air of a natural gas burner, significant energy savings and NOx reduction can be achieved. However, to remain economical, such air humidifying systems are based on the Steam Pump principle, where energy of the flue gases is used to warm and humidify the combustion air of a burner.

In the state of the art, such systems are limited to natural gas burners, because of the high water content of the flue gases. The condensate obtained from cooling down these flue gases is around 140° C. This condensate is then used to warm and humidify the intake air of a burner. The energy transfer is limited by that temperature.

Coal, however, produces flue gases with much less water than natural gas, and the resulting dew point is too low to use the steam pump design.

It is therefore highly desirable to have a boiler where a heat recovery system is used to warm and humidify a flow of intake air to the burner.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a water heater system comprising: a hot water boiler having top, bottom and side walls; a flue connected to the top wall; a burner secured to a side wall of the first housing; a combustible feeder connected to the burner; an evaporator having a housing comprising an outlet, heat exchange elements located in the housing and a water discharge device spaced above the heat exchange elements, wherein the evaporator provides a source of humid air to the burner for increasing the combustion products dew point and to reduce NOx emissions when burned; and a heat recovery system connected to the flue wherein the heat is used for heating water used by the water discharge device.

In a specific embodiment of the present invention, the water heater system further comprises a flue gas analyzer connected to the flue and to the outlet of the evaporator for measuring the level of at least one of CO₂, thermal and fuel NOx and H₂O and for measuring the temperature and water content at the outlet of the evaporator; and a controller for analyzing the values obtained from the flue gas analyzer, wherein if at least one of these values, alone or in combination, indicates a sub-optimal combustion condition, the controller adjust the operation parameters of the water heater system to reach optimal combustion condition.

In a preferred embodiment of the present invention, the heat recovery system is a source of hot water such as an indirect economizer. The right amount of water is taken from the heat recovery system based on controller output. It is also understood that other types of heat recovery systems would be suitable for the present invention and that water could be heated separately from the heat coming from the heat recovery system.

This system could be used replacing the hot water boiler with other types of heating device such as steam boilers and cogen units.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects and advantages of the present invention will become better understood with reference to the description in association with the following in which:

FIG. 1 is a simplified schematic diagram of the system of an embodiment of the present invention;

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a water heater system in which hot water from a heat recovery system is used to warm and humidify a flow of intake air to the burner. The objectives are to reduce energy consumption and to reduce flame temperature so thermal and fuel NOx's will be reduced. An advantage of the present invention is that the amount of water and the temperature of the air are closed loop controlled to the maximum level before negative effects appear.

The following parameters play a role: condensation of water in the air piping, with presence of water droplets in the air; reduction of the O₂ availability to sustain the combustion; and presence of combustion residues like soot.

The major source of NOx production from nitrogen-bearing fuels such, as certain coals and oil, is the conversion of fuel bound nitrogen to NOx during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical, and ultimately forms free N2, or NO. Fuel NOx can contribute as much as 50% of total emissions when burning oil and as much as 80% when burning coal. By humidifying the combustion air, we are significantly reducing the amount of fuel bound NOx converted to free N₂ or NO.

Control strategy:

The objective of the system is to ensure that the moisture content of the combustion air of coal, oil or any biomass burner air is kept at maximum practical level because the water vapor in the combustion air creates conditions that improve gasification.

Gasification is a process that converts solid carbonaceous materials (coal, petroleum products, or biomass) by thermochemical reactions into a fuel gas known as syngas, which is rich in hydrogen and carbon moNOxide. The process needs to operate with an oxidizer (air, oxygen, steam or a combination) under sub-conditions. However, usually air and steam mixture are commonly used oxidizer in industry. Then, the syngas can be directly fired into gas turbines or boilers. The overall process is done in several processes and zones:

-   A) Pyrolysis -   B) Oxidation -   C) Gasification & hydrogenation

The pyrolysis process occurs as the solid carbonaceous material heats up (302-1292° F.) in the absence of oxygen to release volatiles (tar, hydrogen, and carbon moNOxide) and produce char. The weight loss of the solid materials depends on its volatiles content and also on the operating conditions.

In the oxidation zone, the char and some of the released volatiles go through the following exothermic oxidation reactions (1292-3632° F.):

C+O₂

CO₂   (1)

C+½O₂

CO   (2)

CO+½O₂

CO₂   (3)

2H₂+O₂

2H₂O   (4)

Where C represents the carbon-containing solid and/or char.

When the gasification and hydrogenation step occurs, combusted and uncombusted products as well as water vapor pass through a charcoal bed where the following reactions take place (1472-2012° F.):

C+CO₂

2CO   (5)

“Boudouard reaction: endothermic”

C+H₂O

H₂+CO   (6)

“water-gas reaction: endothermic”

CO+H₂O

CO₂+H₂   (7)

“water shift reaction: exothermic”

C+2H₂

CH₄   (8)

“methanation: exothermic”

It is important to note that the reversible gas phase water gas shift reaction reaches equilibrium very fast at the temperatures of a gasifier, which as a result should balance the concentrations of carbon moNOxide, carbon dioxide, and hydrogen.

The aim is to use the gasification air humidification to enhance the overall gasification process, hence increasing the quantity and the high heating value of the generated syngas.

In order to keep that process under optimal control, two parameters are monitored: the flame temperature and the CO level of the flue gas. As the moisture content of the combustion air increases, the flame temperature goes down, and the CO content goes up.

The flame temperature is estimated from the flue gas temperature. The CO level is measured from the flue gas. If the flue gas temperature goes below a predetermined value, while maintaining the CO concentration at the limit of 400 PPM, it is an indication that the combustion process is adversely affected by the high water content of the combustion air.

In parallel to controlling these two output parameters, the control system will monitor the following input parameters:

1. Ambient air temperature and humidity ratio.

2. Water inlet flow & temperature. We need to measure the water inlet/outlet flows and temperatures to control the level of combustion air humidification in the evaporator.

3. Saturated combustion air temperature leaving evaporator.

When temperature or CO content of the flue gas indicates sub-optimal combustion conditions, the control system will change the above inputs (points 1 to 3) to reestablish optimal combustion conditions. The commands of the control system will be based on the level of these 3 input parameters, according to the following priorities.

Referring now to FIG. 1, there is shown at (10) an embodiment of the water heater system of the present invention. The system (10) comprises an evaporator (20) having a vertical cylindrical housing (22). Heat exchange elements (24) are provided inside the housing (22). Water is sprayed through a water discharge device (26) on top of the heat exchange elements (24) and drop down the housing (22). The humidified air is evacuated by an outlet (28) to reach the burner (38) of the hot water boiler (30). The downward flow of hot water exchanges heat with the upward flow of air provided by an air inlet (29). Various techniques can be used to improve heat exchange performance like packing, number of spraying head, size of the water particulates, etc. Since the air is saturated, and the dew point is about 190° F., a reheat coil is used to further warm the air, preventing water from condensing in the piping or boiler elements.

The hot water boiler (30) comprises top and bottom walls (32) (34) and side walls (36) and the burner (38) is secured to one of the side walls (36). A combustible feeder (39) is attached to the burner (38) for providing the combustible needed. The hot water boiler (30) further comprises a flue (40) for evacuating the flue gas. To the flue (40) is connected a heat recovery system (42) which is an indirect economizer and is used as a source of hot water.

The flue gas then passes through a flue gas analyzer (50) which measures the level of CO₂, thermal and fuel NOx, H₂O and any other parameter of the flue gas. It also measure the temperature and water content at the outlet (28) of the evaporator (20). The flue gas analyzer (50) is connected to a controller (60). The controller (60) uses the information from the flue gas analyzer (50) to determine the proper amount of hot water to feed to the evaporator (20), based on water temperature, and quality of the combustion. The control algorithm (fuzzy logic or else) keeps in optimal operating conditions for maximum energy savings, reducing pollutant emissions, with the maximum amount in the air without reducing burner efficiency. 

1. A water heater system comprising: a hot water boiler having top, bottom and side walls; a flue connected to said top wall; a burner secured to one side wall; a combustible feeder connected to said burner; an evaporator having a housing, said housing comprising an outlet, heat exchange elements located in said housing and a water discharge device spaced above said heat exchange elements, wherein said evaporator provides a source of humid air to said burner for increasing the combustion product dew point and to reduce NOx emission when burned; and a heat recovery system connected to said flue wherein said heat is used for heating water used by said water discharge device.
 2. The system of claim 1, further comprising a flue gas analyzer connected to said flue and to the outlet of said evaporator for measuring the level of at least one of CO₂, thermal and fuel NOx and H₂O and for measuring the temperature and water content at said outlet of said evaporator; and a controller for analyzing said level of at least one of CO₂, thermal and fuel NOx and H₂O and said temperature and water content at said outlet of said evaporator, wherein if at least one of these values, alone or in combination, indicate a sub-optimal combustion condition, said controller adjusts the operation parameters of said water heater system to reach optimal combustion condition. 